1. Field of the Invention
The present invention relates to a permanent magnet type three-phase ac generator that is mounted on a motor cycle to charge a battery and to supply electric power to a head lamp or an electric load.
2. Description of the Related Art
Recently, it has become necessary for a permanent magnet type three-phase ac generator to meet demands for size-reduction and power increase at a lower speed range.
In general, as the size of the generator is reduced, spaces for armature windings are also reduced. As a result, large-sized coils can not be mounted in a conventional permanent magnet type three-phase ac generator. Accordingly, the output current of at a higher speed range unnecessarily increases if the output current at a lower range is increased. This increased output power heats the coils up to an excessively high temperature.
JP-A 2001-112226 or its counterpart U.S. Pat. No. 6,573,629 discloses a permanent magnet type three-phase ac generator that has a rotor with 16 magnetic poles and a stator with 18 teeth, as shown in FIG. 10. The stator includes a three-phase armature winding having three phase-windings. Each phase-winding is composed of 6 coils connected as shown in FIG. 7. The rotor with 16 magnetic poles increases the frequency of alternating magnetic field, which increases the output voltage at a lower speed range and suppresses the output current at a higher speed range. Therefore, the size of the permanent magnet per pole can be reduced, and the temperature of the coils, which composes the armature winding, can be reduced. However, because there is a phase difference of 20 degrees in voltage induced in coils of the same phase-winding as shown in FIG. 8, the output voltage induced in the same phase-winding is suppressed. This is an obstacle of size reduction of the permanent magnets. The above structure also necessitates long wires to connect the coils in order to minimize differences in phase.